A coating composition comprising a thermoplastic resin such as a vinyl chloride-vinyl acetate copolymer, a vinyl chloride-vinylidene chloride copolymer, a cellulose resin, an acetal resin, a urethane resin or an acrylonitrile-butadiene copolymer alone or in combination as a binder and a ferromagnetic powder coated on a support such as a polyethylene terephthalate, cellulose triacetate or a polyvinyl chloride has been used for a magnetic recording medium. However, the wear resistance of the magnetic recording layer of the magnetic recording medium is worse and tape guiding systems for running the magnetic tapes are readily contaminated, as the more the medium is used.
In order to prepare a magnetic recording layer having excellent wear resistance, a thermosetting resin (such as a melamine resin or a urea resin) or a mixture of the thermosetting resin and a cross-linkable binder which is cross-linked by chemical reaction, such as an isocyanate compound or an epoxy compound is used as a binder as described in, for example, U.S. Pat. Nos. 3,419,420, 3,499,789, 3,597,273, 3,713,887, 3,781,210 and 3,781,211..
A urea formation reaction by which urea is formed from an isocyanate group and water, a biuret formation reaction in which urea, formed by the urea formation reaction, is reacted with an isocyanate group, a urethane formation reaction in which an isocyanate group is reacted with a hydroxy group in another binder, an allophanate formation reaction in which a urethane bond is reacted with an isocyanate group, an amide formation reaction in which a carboxylic group in another binder is reacted with an isocyanate group, and an acylurea formation reaction in which an amide is reacted with an isocyanate group are known as reactions of polyisocyanate in a magnetic recording layer. The curing reaction proceeds in a magnetic recording layer in accordance with the above-described reactions.